This invention relates generally to radio detecting and ranging (radar) systems and, more specifically, to processing radar signals to automatically detect targets.
Currently developed automatic radar target detection systems generally lack accuracy for target detection. In particular, space-based radar systems may miss many targets or, on the other hand, may yield high false alarm rates. This problem is particularly acute in detecting slow-moving vehicles because the slow velocity of these vehicles is not as readily distinguishable from Doppler radar processing effects on radar echoes than is the case with faster moving vehicles. Accordingly, currently developed automatic detection systems have minimum detectable velocity (MDV) ratings below what is desired. Generally speaking, current radar transducing technology is capable of providing the data needed to manually and automatically detect targets with greater accuracy. However, methods and systems used to process that data cannot automatically detect targets with sufficient accuracy.
As a result of the shortcomings of automatic target detection, aerial radar platforms currently relay image data to ground stations where human analysts manually inspect the image data for targets. This process is costly in many ways. Manual verification necessitates increased transmission bandwidth to get the image data to the human analysts. Further, a staff of analysts presents considerable manpower and facilities costs. Moreover, this costly process is time-consuming, and therefore undermines the value of the resulting analysis. By the time an analyst receives, reviews, and renders a conclusion, the target may have moved, possibly out of range of further tracking and/or prosecution.
Current automatic target detection research tends to concentrate on systems based on single polarization radar systems. Single polarization radar provides only a single set of planar transmit and receive data. Thus, it is more manageable for processing purposes than multiple polarization radar. Respecting limits of on-board processing systems of radar platforms, therefore, much research has been concentrated in how to better process single polarization radar data to identify targets.
Unfortunately, currently developed single polarization radar processing techniques have yet to yield satisfactory results. For example, the Defense Advanced Research Projects Agency (DARPA) has set as a goal having a detection probability rate of 98 percent, while having a false alarm rate on the order of 0.001 false alarms per square kilometer. However, current automatic target detection systems using single polarization radar with adequate sensitivity generally have false alarm rates of about 2 false alarms per square kilometer. This false alarm rate is orders of magnitude higher than is desired.
Multiple polarization radar has the potential to enhance automatic detection of targets. Multiple polarization radar transmits and receives signals in both vertical and horizontal planes. Thus, multiple polarization radar yields four sets of data. These sets include two forms of single polarization data: vertical transmit/receive data and horizontal transmit/receive data. These sets also include two forms of cross polarization data: vertical transmit/horizontal receive data and horizontal transmit/vertical receive data. Accounting for the varied alignment and resulting reflection of signals by differently oriented targets, multiple polarization radar can potentially detect targets that single polarization radar might not. The wealth of data returned by multiple polarization radar also demands greater processing resources.
In an attempt to exploit multiple polarization radar without exceeding available on-board processing capabilities, current multiple polarization radar automatic detection systems have attempted to limit their processing to one or more yielded parameters, such as radar cross section. Unfortunately, limiting the processing to a single quantity has not resulted in the type of automatic detection accuracy desired. Similarly, combinations of quantities researched to date also have failed to result in desired accuracy within the capabilities of available processing systems.
Thus, there is an unmet need in the art for an automatic target detection algorithm that takes advantage of the data provided by multiple polarization ground moving target indicator radar to yield greater sensitivity and low false alarm rates, particularly in detecting slow-moving targets.
The present invention provides a system for processing radar signals to more accurately identify targets of interest, particularly moving targets. From data yielded by multiple polarization radar and filtered to enhance moving target signatures and suppress clutter background, targets can be detected with improved accuracy using three quantities which can be readily calculated from the multiple polarization radar data. These calculated quantities can be analyzed to determine whether a point evaluated by the radar constitutes a target point or a clutter point.
An exemplary embodiment of the present invention detects targets in radar signals by first calculating, for a working point in a working radar data, a working first-sense circular transmit/first-sense circular receive radar cross-section, a working first-sense circular transmit/second-sense circular receive radar cross-section, and a working asymmetry angle from a working scattering matrix. These calculated quantities are then evaluated to determine if these quantities indicate the working point is a target point or a clutter point. In one embodiment, the working point is evaluated by comparing the calculated quantities to a look-up table. The look-up table classifies a first-sense circular transmit/first-sense circular receive radar cross-section, a first-sense circular transmit/second-sense circular receive radar cross-section, and an asymmetry angle representing target points and clutter points. The look-up table suitably is derived from basis radar data in which these quantities can be previously classified as representing target points or clutter points. Each of the working points is classified as a target point or a clutter point by reading from previously classified combinations of these quantities as listed in the look-up table.
In a preferred embodiment, ground moving target indicator (GMTI) multiple polarization radar is used to collect basis and working data from both vertical and horizontal single polarization planar transmit and receive data, and one or both cross polarization data scans. A look-up table is created empirically from predetermined information about combinations of a first-sense circular transmit/first-sense circular receive radar cross-section, a first-sense circular transmit/second-sense circular receive radar cross-section, and an asymmetry angle representing target points and clutter points, eliminating suspected cultural clutter points as desired.